In the beginning of the master's thesis, an overview of the set up of the buildings cadastere and real estate land register in Slovenia is given. The overall view of the activities in the set-up field of these two registers is followed by the presentation of the actual data model of the real estate register (REN). The thesis especially emphasises the analysis of the data model REN and the studies of the data needed for individual models of mass assessment of general market value of real estate. The manner of data changes in REN is presented in more detail. The steps of value calculations are described for the chosen models of mass assessment i.e. apartments, houses, premises and offices and the equations used to calculate the general market value are given. The practical part of the thesis contains the description of the data model REN including the graphic presentation of the connections between the individual dependent tables of the data collection. Study examples have been chosen to show examples of mistakes in REN. Data changes with the study report (for cadastre registration record) are presented for the chosen examples accompanied by the simplified manner of registration record. In the conclusion, the findings on the imperfection of the data collection REN are given together with suggestions how to improve the quality of real estate data.